Process for producing sulfur-curable acrylic rubbers

ABSTRACT

A process for producing a sulfur-curable acrylic rubber is provided which comprises radical-copolymerizing (1) an alkyl acrylate with (2) an alkenyl acrylate or methacrylate, or radical-copolymerizing (1) an alkyl acrylate, (2) an alkenyl acrylate or methacrylate, (3) an alkoxyalkyl acrylate and optionally (4) acrylonitrile. These acrylic rubbers can be cured at a rate as fast as those for curing diene-type rubbers, and the cured products exhibit superior properties without long heat-treatment after curing.

This invention relates to a process for producing an acrylic rubber which can be cured with sulfur.

Acrylic rubber is an elastomeric copolymer composed mainly of an acrylate unit, and is known to have superior heat resistance and oil resistance. Since it does not have a double bond at the main chain of the rubber molecules, a monomeric component having an active group capable of becoming a crosslinking site is usually copolymerized with it.

Monomers previously used as the monomeric component for providing crosslinking sites include, for example, halogen-containing monomers such as 2-chloroethyl vinyl ether, vinyl benzyl chloride, vinyl chloroacetate, allyl chloroacetate, and 5-chloroacetoxymethyl-2-norbornene, and epoxy monomers such as allyl glycidyl ether, glycidyl acrylate and glycidyl methacrylate.

Because these acrylic rubbers cannot be cured with ordinary sulfur (or sulfur-containing organic compound), vulcanization accelerator systems, amines, ammonium salts, metal soap/sulfur, etc. are generally used as a vulcanizer. Curing with these vulcanizers, however, has the defect that the rate of cure is slow, and to obtain vulcanizates of good properties, the cured products must be heat-treated for a very long period of time after curing.

Furthermore, when the amines or ammonium salts are used as the vulcanizer, unpleasantly odoriferous gases are generated in places where rubber products are made, thus causing a sanitary hazard. This also causes the defect that the cured products cannot be used in medical and foodstuff applications.

The use of the halogen-containing monomers as the crosslinking monomeric component may cause the corrosion of the mold at the time of curing, or the corrosion of metals with which the vulcanized products will make contact.

In an attempt to remove these defects, a method was suggested which involves copolymerization of an acrylate ester with such a crosslinking monomeric component as dicyclopentadiene, methylcyclopentadiene, ethylidene norbornene, vinylidene norbornene, butadiene, isoprene, allyl acrylate, 2-butenyl acrylate, methallyl acrylate, or triallyl isocyanurate to produce an acrylic rubber. The acrylic rubbers obtained by such a method are not entirely satisfactory for practical application because of one or more disadvantages. For example, the rate of cure is extremely slow, or the properties of the cured product are far from meeting the requirements of practical application. Or although the aforesaid problems are solved to some extent, the heat resistance and compression set of the products are still inferior to acrylic rubbers obtained by using the halogen-containing monomers or epoxy monomers as the crosslinking monomeric component.

It is an object of this invention therefore to provide sulfur-curable acrylic rubbers having superior compression set, which permit an improved rate of cure as fast as that for curing natural rubbers and diene-type synthetic rubbers and do not require long heat-treatment after curing, without sacrificing the superior heat resistance, oil resistance, weatherability and ozone resistance of conventional acrylic rubbers.

We have now found that this object can be achieved by using a certain alkenyl acrylate and/or an alkenyl methacrylate as a crosslinking component.

Thus, according to this invention, there is provided a process for producing a sulfur-curable acrylic rubber, which comprises copolymerizing in the presence of a radical initiator (1) 90 to 99.5% by weight of at least one alkyl acrylate with the alkyl group containing 1 to 8 carbon atoms and (2) 0.5 to 10% by weight, preferably 2 to 6% by weight, of at least one monomer of the general formula ##STR1## wherein R₁ represents a hydrogen atom or a methyl group, and R₂ and R₃, independently from each other, represent an alkyl group having 1 to 3 carbon atoms.

We have also found that when a part of the alkyl acrylate (1) is replaced by another monomer in the aforesaid process for producing acrylic rubber, there can be obtained an acrylic rubber which has a well balanced combination of cold resistance and oil resistance in addition to the aforesaid favorable properties.

Thus, according to another aspect, there is provided a process for producing a sulfur-curable acrylic rubber, which comprises copolymerizing in the presence of a radical initiator (1) 30 to 89.5% by weight of at least one alkyl acrylate with the alkyl group containing 1 to 8 carbon atoms, (2) 0.5 to 10% by weight, preferably 2 to 6% by weight, of at least one monomer of the general formula ##STR2## wherein R₁ represents a hydrogen atom or a methyl group, and R₂ and R₃, independently from each other, represent an alkyl group containing 1 to 3 carbon atoms,

(3) 10 to 60% by weight, preferably 20 to 50% by weight, of at least one alkoxyalkyl acrylate with the alkoxy group containing 1 to 4 and the alkylene group containing 1 to 4 carbon atoms, and (4) 0 to 30% by weight, preferably 2 to 20% by weight, of acrylonitrile.

Examples of the alkyl acrylate (1) are methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, amyl acrylate, hexyl acrylate, cyclohexyl acrylate, and octyl acrylate. Those in which the alkyl group contains 2 to 4 carbon atoms are preferred; for example, they are ethyl acrylate, propyl acrylate and butyl acrylate.

Examples of the alkenyl acrylate and alkenyl methacrylate (2) of general formula (I) include 3-methyl-2-butenyl acrylate, 3-methyl-2-pentyl acrylate, and 3-methyl-2-hexenyl acrylate, and the corresponding methacrylates.

Acrylic rubbers made by the copolymerization of allyl acrylate or 2-butenyl acrylate with alkyl acrylates are known (U.S. Pat. No. 3,476,722). These acrylic rubbers, however, require long heat-treatment after curing as is the case with the conventional acrylic rubbers using the halogen-containing monomers or epoxy monomers as the crosslinking monomeric component, and the properties of the vulcanizates are not satisfactory for practical purposes. Moreover, acrylic rubbers obtained by the ternary copolymerization of allyl methacrylate, an alkyl acrylate and acrylonitrile are also known (Japanese Patent Publication No. 7893/72). But these acrylic rubbers have poor mechanical strength, and cannot find practical application.

When in accordance with this invention, at least one monomer of general formula (I) (i.e., an alkenyl acrylate or alkenyl methacrylate) is used as the crosslinking monomer, these defects can be markedly remedied. If the amount of the monomer (2) is less than 0.5% by weight, the rate of cure is extremely low, and vulcanizates having satisfactory properties for practical application cannot be obtained. If the amount of the monomer (2) exceeds 10% by weight, the cured product has very high hardness, and a greatly reduced elongation, and therefore, cannot be used satisfactorily as a rubber product.

Examples of the alkoxyalkyl acrylate (3) include methoxymethyl acrylate, methoxyethyl acrylate, ethoxyethyl acrylate, and butoxyethyl acrylate.

The acrylonitrile (4) is used in the aforesaid amounts when it is necessary to adjust the strength and oil resistance of the acrylic rubber of this invention.

The process of this invention can be easily performed by polymerizing a mixture of the monomers (1) and (2) or a mixture of the monomers (1), (2) and (3) and optionally (4) in the presence of a radical initiator by a known polymerization method (e.g., in emulsion, suspension, solution, bulk, etc.). The polymerization is performed batchwise, or while adding at least one component continuously and/or intermittently during the reaction. The polymerization temperature is preferably from -10° C. to +100° C., more preferably from 2° to 80° C.

The resulting acrylic rubber can be easily cured with sulfur vulcanization systems generally used for natural rubbers or diene-type synthetic rubbers, and this curing treatment does not require the very long heat-treatment after curing which is essential in the curing of ordinary acrylic rubbers having a halogen containing monomer or epoxy monomer as crosslinking sites. The cured products have superior heat resistance, weatherability and ozone resistance, and in some cases, a well balanced combination of cold resistance and oil resistance. Furthermore, the acrylic rubbers of this invention have markedly improved heat resistance and compression set over the known sulfur-curable acrylic rubbers.

In addition to the vulcanization system, various compounding agents such as reinforcing agents, fillers, plasticizers and antioxidants may be added to the rubbers of this invention as required.

The cured products of the acrylic rubbers of this invention are useful in many applications which require heat resistance, oil resistance, weatherability and ozone resistance. They include, for example, various rubber products such as gaskets, hoses, conveyor belts, packings, oil seals and valve seats.

The present invention is more specifically illustrated below with reference to Examples and the accompanying drawings.

In the drawings,

FIG. 1 shows cure curves of acrylic rubbers obtained in Runs Nos. 3, 9 and 10 in Example 1 which were measured by means of an oscillating disc rheometer (a product of Toyo Seiki K.K.); and

FIG. 2 show cure curves of acrylic rubbers obtained in Runs Nos. 1, 7 and 8 of Example 2 measured in the same way as described above. In these graphic representations, the abscissa represents the cure time (minutes), and the ordinates, the torque (kg.cm). The solid line refers to the example within the scope of this invention, and the broken lines, to comparisons.

EXAMPLE 1

A series of copolymers were produced by using the monomeric mixtures shown in Table 1. The method of polymerization was as follows:

A 2-liter separable flask equipped with a thermometer, a stirrer, a nitrogen introducing tube and an evacuation device was charged with a mixture of the following formulation.

    ______________________________________                                         Water                    1,000 g                                               Sodium dodecylbenzenesulfonate                                                                          20 g                                                  Sodium naphthalenesulfonate                                                                             10 g                                                  Sodium sulfate           3 g                                                   Tetrasodium ethylenediamine-                                                   tetraacetate             0.2 g                                                 Ferrous sulfate          0.005 g                                               Monomeric mixture (Table 1)                                                                             1000 g                                                ______________________________________                                    

The pH of the mixture was adjusted to 7, and with stirring, the temperature of the inside of the flask was maintained at 5° C. Furthermore, the inside of the flask was deoxygenated fully by using the nitrogen introducing tube and the evacuation device. Then, the following compounds were added, and the reaction was started.

Na₂ S₂ O₄ :0.2 g

Sodium formaldehyde sulfoxylate: 0.2 g

p-Menthane hydroperoxide: 0.1 g

The polymerization was terminated in about 16 hours. The polymerization conversion was within the range of 95 to 99% in each run. After the polymerization, the reaction mixture was salted out, washed with water, and dried under vacuum in a dryer to form the desired acrylic rubber.

                  TABLE 1                                                          ______________________________________                                         Monomers     Run No                                                            (parts by    Invention     Comparison                                          weight)      1     2     3   4   5   6   7   8   9   10                        ______________________________________                                         Ethyl acrylate                                                                              99    97    95  97  32  97  97  97  98  95                        Butyl acrylate                                                                              --    --    --  --  65  --  --  --  --  --                        3-Methyl-2-butenyl                                                             acrylate      1     3     5  --   3  --  --  --  --  --                        Allyl acrylate                                                                              --    --    --  --  --   3  --  --  --  --                        2-Butenyl acrylate                                                                          --    --    --  --  --  --   3  --  --  --                        Methallyl acrylate                                                                          --    --    --  --  --  --  --   3  --  --                        Glycidyl                                                                       methacrylate --    --    --  --  --  --  --  --   2  --                        2-Chloroethylvinyl                                                             ether        --    --    --  --  --  --  --  --  --   5                        ______________________________________                                    

Using the 10 acrylic rubbers obtained, compounds were prepared by means of an open roll in accordance with the recipes shown in Table 2. The compounds were each press-cured at 160° C. for 20 minutes, and then heat-treated in a Geer oven at 150° C. for 4 hours and 16 hours, respectively.

                  TABLE 2                                                          ______________________________________                                         Recipe          Run No.                                                        (parts by weight)                                                                              1-8       9       10                                           ______________________________________                                         Acrylic rubber  100       100     100                                          Stearic acid    1         1       1                                            HAF-LS carbon black                                                                            60        60      60                                           Zinc oxide No. 1                                                                               3         --      --                                           Red lead        --        --      5                                            2-Mercaptobenzimida-                                                           zole            --        --      1.5                                          Ammonium benzoate                                                                              --        1       --                                           Tetramethylthiuram                                                             disulfide       2         --      --                                           4,4'-Dithiomorpholine                                                                          2         --      --                                           N--cyclohexyl-2-benzo-                                                         thiazyl sulfenamide                                                                            1         --      --                                           ______________________________________                                    

The properties of the cured products were measured in accordance with JIS K-6301, and the results are shown in Table 3.

The cure behaviors of a compound containing the acrylic rubber of this invention (Run No. 3) and compounds containing conventional acrylic rubbers including an epoxy monomer and a halogen-containing monomer as the crosslinking monomer (Runs Nos. 9 and 10) were measured by an oscillating disc rheometer. The relation between the curing time and the torque were plotted in FIG. 1.

                                      TABLE 3-1                                    __________________________________________________________________________                   Run No.                                                                        Invention      Comparison                                        Test items    1  2  3  4  5  6  7  8  9  10                                    __________________________________________________________________________     Properties after curing                                                        at 160° C. for 20 minutes                                               Tensile strength (kg/cm.sup.2)                                                               121                                                                               153                                                                               152                                                                               111                                                                               140                                                                               64 73 70 106                                                                               98                                    100% Tensile stress (kg/cm.sup.2)                                                            11 36 49 18 31 11 15 13 40 19                                    Elongation (%)                                                                               710                                                                               330                                                                               190                                                                               510                                                                               340                                                                               320                                                                               300                                                                               300                                                                               340                                                                               490                                   Hardness (Shore A)                                                                           50 67 69 54 65 48 52 50 63 58                                    Compression set (%)(*)                                                                       69 54 55 76 68 84 80 83 80 89                                    __________________________________________________________________________      (*): Compression ratio 25%, 150° C. × 70 hours              

                                      TABLE 3-2                                    __________________________________________________________________________                   Run No.                                                                        Invention      Comparison                                        Test items    1  2  3  4  5  6  7  8  9  10                                    __________________________________________________________________________     Properties after curing at 160° C. for 20 minutes and then              heat-treatment at 150° C. for 4 hours                                   __________________________________________________________________________     Tensile strength (kg/cm.sup.2)                                                               134                                                                               149                                                                               149                                                                               120                                                                               139                                                                               67 81 73 113                                                                               119                                   100% Tensile stress (kg/cm.sup.2)                                                            14 36 49 19 36 15 23 20 53 31                                    Elongation (%)                                                                               600                                                                               320                                                                               190                                                                               470                                                                               310                                                                               270                                                                               280                                                                               270                                                                               290                                                                               350                                   Hardness (Shore A)                                                                           54 67 69 57 67 54 58 55 66 62                                    Compression set (%)(*)                                                                       30 27 22 38 34 63 56 61 56 77                                    __________________________________________________________________________     Properties after curing at 160° C. for 20 minutes and then              heat-treatment at 150° C. for 16 hours                                  __________________________________________________________________________     Tensile strength (kg/cm.sup.2)                                                               Since the properties reached                                                                  69 70 68 134                                                                               137                                   100% Tensile stress (kg/cm.sup.2)                                                            an equilibrium as a result of                                                                 21 29 25 69 62                                    Elongation (%)                                                                               the curing at 160° C. for 20                                                           230                                                                               220                                                                               220                                                                               210                                                                               220                                   Hardness (Shore A)                                                                           minutes and heat treatment at                                                                 59 61 60 69 69                                    Compression set (%)(*)                                                                       150° C. for 4 hours, the                                                               51 44 45 29 59                                                  testing of the properties                                                      after the 16-hours heat-                                                       treatment was omitted                                            __________________________________________________________________________

                                      TABLE 3-3                                    __________________________________________________________________________                  Run No.                                                                        Invention           Comparison                                    Test items   1   2   3   4   5   6   7   8   9   10                            __________________________________________________________________________     Heat aging test (in Geer oven at 175° C. for 170 hours) (**)            Percent change in tensile                                                      strength (%) -47 -28 -4  -32 -27 -48 -26 -41 +15 -37                           Percent change in elonga-                                                      tion (%)     +8  ±0                                                                              -8  -15 -7  -48 -32 -50 -20 -43                           Change in hardness (point)                                                                  +12 +11 +12 +15 +13 +20 +18 +20 +9  +8                            Heat aging test in oil (150° C. for 70 hours in ASTM No. 3 oil)         (**)                                                                           Percent change in volume                                                       (%)          +17.0                                                                              +15.1                                                                              + 13.6                                                                             +16.2                                                                              +26.3                                                                              +16.5                                                                              +16.0                                                                              +16.5                                                                              +16.7                                                                              +17.8                         Cold resistance test (low-temperature torsion test in JIS K-6301 (**)          T-10 (°C.)                                                                           -12.0                                                                              -10.0                                                                              -9.5                                                                               -10.0                                                                              -21.0                                                                              -10.0                                                                              -10.0                                                                              -10.0                                                                              -10.0                                                                              -10.0                         __________________________________________________________________________      (**): In Runs Nos. 1 to 8 samples cured at 160° C. for 20 minutes       (not heattreated subsequently) were used, and in Runs Nos. 9 and 10,           samples cured at 160° C. for 20 minutes and then heattreated at         150° C. for 16 hours were used.                                   

It is seen from FIG. 1 that while the conventional acrylic rubbers (Runs Nos. 9 and 10) cure very slowly, the acrylic rubber of this invention (Run No. 3) cures very rapidly after starting of vulcanization and then reaches an equilibrium, thus exhibiting the same vulcanization behavior as a diene-type rubber. Hence, the acrylic rubber of the invention shows satisfactory properties for practical application when cured at 160° C. for 20 minutes, and its compression set is improved by heat-treatment at 150° C. for as short as 4 hours. To obtain vulcanizates having properties equivalent to the acrylic rubber of this invention, the conventional acrylic rubbers must be heat-treated at 150° C. for as long as 16 hours.

Acrylic rubbers including allyl acrylate, 2-butenyl acrylate, and methallyl acrylate (Runs Nos. 6, 7 and 8, respectively) have very poor roll processability, and even when heat-treated in the same way as in the case of the conventional acrylic rubbers, cannot have satisfactory vulcanization properties for practical application.

It is appreciated from the experimental results given above that the acrylic rubber of this invention is an acrylic rubber of very good quality which shows equivalent or better vulcanization properties to or than the conventional acrylic rubbers without performing the long heat-treatment which is essential to the vulcanization of the conventional acrylic rubbers.

EXAMPLE 2

A 2-liter separable flask equipped with a thermometer, a stirrer, a nitrogen introducing tube and an evacuation device was charged with a mixture of recipe (I) below. The pH of the mixture in the flask was adjusted to 7, and with stirring, the temperature of the inside of the flask was maintained at 5° C. The oxygen in the flask was sufficiently removed by repeated deaeration and nitrogen introduction. Then, a mixture of recipe (II) below was added, and the polymerization was started. The polymerization was terminated in about 16 hours. The polymerization conversion was within the range of 95 to 99% in each run. After the polymerization, the product was salted out, washed thoroughly with water, and dried under reduced pressure in a dryer to obtain the desired acrylic rubber.

Polymerization Recipe

Recipe (I)

    ______________________________________                                         Water                    1000 g                                                Sodium dodecylbenzenesulfonate                                                                          20 g                                                  Sodium naphthalenesulfonate                                                                             10 g                                                  Sodium sulfate           3 g                                                   Tetrasodium ethylenediamine-                                                   tetraacetate             0.2 g                                                 FeSO.sub.4               0.005 g                                               Monomeric mixture (Table 4)                                                                             1000 g                                                ______________________________________                                    

Recipe (II)

    ______________________________________                                         Na.sub.2 S.sub.2 O.sub.4                                                                             0.2 g                                                    Sodium formaldehyde sulfoxylate                                                                      0.2 g                                                    p-Menthane hydroperoxide                                                                             0.1 g                                                    ______________________________________                                    

                  TABLE 4                                                          ______________________________________                                                     Run No.                                                            Monomers      Invention       Comparison                                       (parts by weight)                                                                            1     2     3   4   5   6   7     8                              ______________________________________                                         Ethyl acrylate                                                                               53    47    37  --  52  52  98    95                             Butyl acrylate                                                                               25    15    --  47  20  25  --    --                             Methoxyethyl acrylate                                                                        20    35    60  50  20  20  --    --                             Acrylonitrile --    --    --  --   5  --  --    --                             3-Methyl-2-butenyl                                                             acrylate       3     3     3   3   3  --  --    --                             3-Methyl-2-hexenyl                                                             acrylate      --    --    --  --  --   3  --    --                             Glycidyl methacrylate                                                                        --    --    --  --  --  --   2    --                             2-Chloroethylvinyl                                                             ether         --    --    --  --  --  --  --     5                             ______________________________________                                    

Using the 8 acrylic rubbers so obtained, compounds were prepared by means of an open roll under cooling in accordance with the recipes shown in Table 5, and then each press-cured at 160° C. for 20 minutes. Then, the cured products were each heat-treated in a Geer oven at 150° C. for 4 hours, and 16 hours, respectively.

                  TABLE 5                                                          ______________________________________                                                        Run No.                                                                        Invention                                                                              Comparison                                              Recipe (parts by weight)                                                                        1-6       7       8                                           ______________________________________                                         Acrylic rubber   100       100     100                                         Stearic acid     1         1       1                                           HAF-LS carbon black                                                                             60        60      60                                          Zinc oxide No. 1 3         --      --                                          Red lead         --        --      5                                           2-Mercapto benzimidazole                                                                        --        --      1.5                                         Ammonium benzoate                                                                               --        1       --                                          Tetramethyl thiuram disulfide                                                                   2         --      --                                          4,4'-Dithiomorpholine                                                                           2         --      --                                          N--Cyclohexyl-2-benzothiazyl                                                   sulfenamide      1         --      --                                          ______________________________________                                    

The properties of the cured products were measured in accordance with JIS K-6301, and the results are shown in Table 6.

The vulcanization behaviors of a compound containing the acrylic rubber of this invention (Run No. 1) and compounds containing the comparative acrylic rubbers (Runs Nos. 7 and 8) were measured in the same way as in Example 1, and the results were plotted in FIG. 2.

                                      TABLE 6-1                                    __________________________________________________________________________                   Run No.                                                                        Invention                                                        Test items    1   2   3   4   5   6   7   8                                    __________________________________________________________________________     Properties after curing                                                        at 160° C. for 20 minutes                                               Tensile strength (kg/cm.sup.2)                                                               122 126 134 98  148 103 106 98                                   100% Tensile stress (kg/cm.sup.2)                                                            35  34  35  38  37  36  40  19                                   Elongation (%)                                                                               290 300 310 240 320 240 340 490                                  Hardness (Shore A)                                                                           66  66  66  67  67  67  63  58                                   Compression set (%) (*)                                                                      63  68  67  62  59  73  80  89                                   __________________________________________________________________________      (*):Compression ratio 25%, 150° C. × 70 hours               

                                      TABLE 6-2                                    __________________________________________________________________________                   Run No.                                                                        Invention         Comparison                                     Test items    1  2  3  4  5  6  7  8                                           __________________________________________________________________________     Properties after curing at 160° C. for 20 minutes and then              heat-treatment at 150° C. for 4 hours                                   __________________________________________________________________________     Tensile strength (kg/cm.sup.2)                                                               119                                                                               124                                                                               131                                                                               103                                                                               141                                                                               107                                                                               113                                                                               119                                         100% Tensile stress (kg/cm.sup.2)                                                            35 34 34 39 38 39 53 31                                          Elongation (%)                                                                               290                                                                               300                                                                               310                                                                               240                                                                               310                                                                               220                                                                               290                                                                               350                                         Hardness (Shore A)                                                                           66 66 66 67 67 67 66 62                                          Compression set (%) (*)                                                                      30 33 31 37 32 39 56 77                                          __________________________________________________________________________     Properties after curing at 160° C. for 20 minutes and then              heat-treatment at 150° C. for 16 hours                                  __________________________________________________________________________     Tensile strength (kg/cm.sup.2)                                                               Since the properties reached an                                                                  134                                                                               137                                         100% Tensile stress (kg/cm.sup.2)                                                            equilibrium as a result of the                                                                   69 62                                          Elongation (%)                                                                               curing at 160° C. for 20 minutes                                                          210                                                                               220                                         Hardness (Shore A)                                                                           and heat treatment at 150° C. for                                                         69 69                                          Compression set (%) (*)                                                                      4 hours, the testing of the pro-                                                                 29 59                                                        perties after the 16-hour heat-                                                treatment was omitted                                            __________________________________________________________________________

                                      TABLE 6-3                                    __________________________________________________________________________                    Run No.                                                                        Invention               Comparison                              Test items     1   2   3   4   5   6   7   8                                   __________________________________________________________________________     Heat aging test in oil (150° C. for 70 hours in ASTM No. 3 oil)         (**)                                                                           __________________________________________________________________________     Percent change in volume (%)                                                                  +18.9                                                                              +13.8                                                                              +8.5                                                                               +22.4                                                                              +14.2                                                                              +19.7                                                                              +16.7                                                                              +17.8                               __________________________________________________________________________     Cold resistance test (low-temperature torsion test in JIS 6301)                __________________________________________________________________________     (**)                                                                           T-10 (°C.)                                                                             -21 -21.5                                                                              -23.5                                                                              -34.5                                                                              -16 -21.5                                                                              -10 -10                                 __________________________________________________________________________      (**): In Runs Nos. 1 to 8, samples cured at 160° C. for 20 minutes      (not heattreated subsequently) were used, and in Runs Nos. 9 and 10,           samples cured at 160° C. for 20 minutes and then heattreated at         150° C. for 16 hours were used.                                   

It is seen from FIG. 2 that while the conventional acrylic rubbers cure slowly, the acrylic rubber of this invention exhibits a vulcanization behavior equivalent to a diene-type rubber. Accordingly, as shown in Table 6, while the conventional acrylic rubbers require heat-treatment at 150° C. for as long as 16 hours after the curing, the acrylic rubber of this invention affords cured products having equivalent characteristics to the conventional acrylic rubbers by heat-treatment at 150° C. for as short as 4 hours. Moreover, the acrylic rubbers of this invention have markedly improved heat resistance and compression set over the conventional sulfur-curable acrylic rubbers, and exhibit a well balanced combination of cold resistance and oil resistance.

According to this invention, therefore, the slow cure rate of the conventional acrylic rubbers can be increased to a level equivalent to that of diene-type rubbers without impairing the good heat resistance, oil resistance, weatherability, ozone resistance and cold resistance of the conventional acrylic rubbers, and acrylic rubbers having a low compression set and a well balanced combination of cold resistance and oil resistance can be obtained. 

What we claim is:
 1. A process for producing a sulfur-curable acrylic rubber, which comprises copolymerizing in the presence of a radical initiator(1) 90 to 99.5% by weight of at least one alkyl acrylate with the alkyl group containing 1 to 8 carbon atoms, and (2) 0.5 to 10% by weight of at least one monomer of the general formula ##STR3## wherein R₁ represents a hydrogen atom or a methyl group, and R₂ and R₃, independently from each other, represent an alkyl group containing 1 to 3 carbon atoms.
 2. A process for producing a sulfur-curable acrylic rubber, which comprises copolymerization in the presence of a radical initiator(1) 30 to 89.5% by weight of at least one alkyl acrylate with the alkyl group containing 1 to 8 carbon atoms, (2) 0.5 to 10% by weight of at least one monomer of the general formula ##STR4## wherein R₁ represents a hydrogen atom or a methyl group, and R₂ and R₃, independently from each other, represent an alkyl group containing 1 to 3 carbon atoms, (3) 10 to 60% by weight of at least one alkoxyalkyl acrylate with the alkoxy group containing 1 to 4 carbon atoms and the alkylene group containing 1 to 4 carbon atoms, and (4) 0 to 30% by weight of acrylonitrile.
 3. The process of claim 1 or 2 wherein the monomer (2) represented by the general formula is 3-methyl-2-butenyl acrylate.
 4. The process of claim 1 or 2 wherein the monomer (2) represented by the general formula is 3-methyl-2-butenyl methacrylate.
 5. The process of claim 1 or 2 wherein the copolymerization is carried out in emulsion.
 6. The process of claim 1 or claim 2 wherein the alkyl group of the alkyl acrylate contains 2 to 4 carbon atoms.
 7. The process of claim 1 or claim 2 wherein the monomer (2) is present in an amount of from about 2% to about 6% by weight.
 8. The process of claim 2 wherein the alkoxyacrylate is present in an amount of from about 20% to about 50% by weight.
 9. The process of claim 2 wherein acrylonitrile is present in an amount of from about 2% to about 20% by weight.
 10. The process of claim 1 or claim 2 wherein the radical initiator is p-menthane hydroperoxide. 